Pathways of fermentation — In many microbial fermentations, pyruvic acid can be reduced to which characteristic end-products?

Introduction / Context:
Pyruvate is the pivotal branching point of central metabolism. Under anaerobic conditions, cells regenerate NAD+ by reducing pyruvate to characteristic fermentation end-products. Recognizing these products is essential for understanding industrial fermentation and food microbiology.

Given Data / Assumptions:

• We are considering classic microbial fermentation, not aerobic respiration.
• Different microbes use distinct terminal steps to reoxidize NADH.
• The item asks for common organic end-products derived from pyruvate reduction.

Concept / Approach:
Two emblematic routes: alcoholic fermentation converts pyruvate to acetaldehyde and then ethanol (e.g., by Saccharomyces), and lactic fermentation reduces pyruvate directly to lactate (e.g., by Lactobacillus, Lactococcus). Glucose, fructose, starch, and cellulose are not pyruvate reduction products. Citric and isocitric acids are TCA intermediates formed in aerobic metabolism, not typical fermentative sinks for NADH regeneration.

Step-by-Step Solution:

Verification / Alternative check:
Fermentation profiles in industrial settings (brewing, dairy fermentations) consistently show ethanol or lactate as the main reduced products of pyruvate.

Why Other Options Are Wrong:

• Glucose/fructose: upstream glycolytic substrates, not pyruvate reduction products.
• Starch/cellulose: polysaccharides, biosynthesis not driven by pyruvate reduction in fermentation.
• Citric/isocitric acid: TCA intermediates formed aerobically, not typical fermentation endpoints.

Common Pitfalls:
Mixing catabolic end-products with anabolic polymers; fermentation focuses on redox balance, not polysaccharide synthesis.

Final Answer:
ethyl alcohol and lactic acid